chore(css): remove unneeded CSS vendor prefixes (#37903)

Removes redundant/obsolete WebKit prefixes:

- `-webkit-mask-*` — duplicate the unprefixed `mask-*` siblings already
present; every supported browser handles unprefixed CSS Masking
longhands.
- `-webkit-overflow-scrolling: touch` — a no-op outside iOS Safari <13.

Browser floor (all support unprefixed `mask`): Chrome 120+, Safari
15.4+, Firefox 53+, and PaleMoon/Goanna (verified: unprefixed `mask`
longhands implemented unconditionally in UXP).

---
This PR was written with the help of Claude Opus 4.8

Co-authored-by: Claude (Opus 4.8) <noreply@anthropic.com>

web_src/css/modules/dropdown.css

@@ -209,7 +209,6 @@ select.ui.dropdown {
   overflow-x: hidden;
   overflow-y: auto;
   backface-visibility: hidden;
-  -webkit-overflow-scrolling: touch;
   border-top-width: 0 !important;
   outline: none;
   margin: 0 -1px;
@@ -379,7 +378,6 @@ select.ui.dropdown {
   overflow-x: hidden;
   overflow-y: auto;
   backface-visibility: hidden;
-  -webkit-overflow-scrolling: touch;
 }
 
 @media only screen and (max-width: 767.98px) {
@@ -653,7 +651,6 @@ select.ui.dropdown {
   overflow-x: hidden;
   overflow-y: auto;
   backface-visibility: hidden;
-  -webkit-overflow-scrolling: touch;
   min-width: 100% !important;
   width: auto !important;
 }